Implantable vestibular prosthetic devices, based on cochlear implants, are an emerging technology currently in limited clinical trials. These devices have significant technical constraints and may have limited ability to restore reflexive eye movements with head motion, and appear promising for improving function in some patients with bilateral vestibular deficits. To date, all available devices only stimulate semicircular canal afferents to restore sensitivity to angular acceleration. The other half of the vestibular system, that responding to linear acceleration, is not addressed by these prostheses. Our hypothesis is that restoration of semicircular canal function can improve a patients? sense of balance by working with residual otolith function. If true, our findings may help select the candidates for vestibular implantation who would be most likely to benefit from this technology. Our methodology is to examine eye movements and perform single unit recordings from macaques with vestibular implants during combined linear acceleration and prosthetic semicircular canal stimulation. Our project has two specific aims. In the first, we will examine if prosthetic canal stimulation, like natural canal stimulation, can enable discrimination between linear acceleration from gravity from acceleration due to translation. The ability to discriminate tilt from translation is a fundamental role of the semicircular canal system and critical to spatial orientation. This first aim will use the differences in eye movements elicited during tilt and translation to discern whether prosthetic canal signals can facilitate discrimination of reorientation due to gravity from translation. In the second aim, we will record single unit vestibular nucleus neuronal activity to compare the interactions of prosthetic semicircular canal signals with natural otolith signals to interactions of natural semicircular canal and otolith signals. These experiments will be the first looking at how prosthetic vestibular signals and natural signals interact in the brain. These experiments should provide insight into how the brain process signals from currently used vestibular prostheses and may lead to improvements in approaches to treatment of vestibular problems.